Glass substrates may be used in a variety of applications, ranging from windows to high-performance display devices. The quality requirements for glass substrates have become more stringent as the demand for improved resolution, clarity, and performance increases. Glass quality may, however, be negatively impacted by various processing steps, from forming the glass melt to final packaging of the glass product. In particular, glass sheets may be rendered unsuitable for use by the presence of bubbles and, in some cases, even a single bubble in the glass sheet.
During the melting process, glass precursor batch materials are mixed together and heated in a melter. The batch materials melt and react, giving off reaction gases, which produce bubbles in the molten glass. The molten glass then undergoes a fining step to remove gas bubbles trapped in the melt. However, such fining steps often require long processing times, high energy expenditure, and/or increased expense, as the reaction gases have a long distance to travel to escape the glass melt. To promote the escape of bubbles from the glass melt, fining is often carried out using long tubes (e.g., several meters long) constructed from materials such as platinum, which can have a considerably large footprint and expense.
In addition to consuming space, energy, and/or capital, current fining processes can also limit the glass compositions that can be effectively melted and fined. For example, to drive the escape of bubbles from the glass melt, fining temperatures at least about 50° C., and sometimes at least about 100° C., in excess of the melting temperature are often used. Thus, upper limits on the attainable fining temperature can impose limitations on glass compositions with higher glass transition temperatures, such as temperatures in excess of about 1600° C. or more.
Fining can promote bubble removal via two processes. Stokes fining occurs when an increase in the glass temperature leads to a lower viscosity of the glass melt. Bubbles can then rise more rapidly through the less viscous glass melt. Chemical fining occurs when an increase in the glass temperature chemically reduces a chemical fining agent such as tin, thus releasing oxygen into the glass, which can then be incorporated into the bubbles. As the bubbles take up excess oxygen they increase in size and rise through the glass melt more easily, sometimes merging with other bubbles and/or collapsing. Fining agents can include tin, arsenic, and antimony, to name a few. Arsenic and antimony are stronger fining agents but may pose safety and environmental hazards and, thus, are less frequently used. Tin oxide is relatively safer, but also has relatively weaker fining power. Moreover, the amount of tin that can be incorporated as a fining agent into the glass batch materials is often limited because elevated levels of tin can lead to the formation of secondary crystals during downstream processing (e.g., on the forming body or isopipe).
Various methods for promoting fining have thus been investigated by Applicant, such as vacuum fining, centrifugal fining, and reabsorption of bubbles via deep melt pools. However, these methods still suffer from one or more drawbacks including high cost and/or reduced effectiveness at higher fining temperatures. Hot spot fining, or the production of local zones of increased temperatures within the flowing glass stream, has also been investigated by Applicant. Hot spot fining can be achieved, for example, using traditional burners, microwaves, ultrasound, etc. Microwave and ultrasonic fining may provide a cost advantage over traditional fining processes but can suffer from poor penetration depths and/or can be impractical to implement. Traditional burners using flame combustion to create hot spots can also have one or more disadvantages, such as the inability to precisely control the temperature of the hot spots. If the flame is not hot enough, a hot spot will not be generated and thus will not drive and/or enhance fining. If the flame is too hot, or if the center of the hotspot is too hot, volatilization of less stable oxides in the melt (e.g., boron) may occur, thus negatively impacting the composition of the final product.
Accordingly, it would be advantageous to provide glass fining processes which have higher throughput and/or lower cost, while also minimizing issues relating to glass quality, such as defects caused by bubbles in the melt. It would also be advantageous to provide glass fining processes and apparatuses suitable for melting specialty glass materials, such as glasses with higher glass transition temperatures.